Brian,
If you don't want to be too practical, and I think that is against the rules in this hobby, a Slinky toy operator is a great crowd pleaser. I have seen several at model engineering shows a dozen years or so ago and they attract interest from everyone from kids to model engineers. I have photos some where, but I have no idea where.
Think of a double platform balance scale with a parallel linkage to keep the platforms level while they move up and down. Then drive the platforms with any kind of odd linkages that you can think of. Belts, gears, bell cranks, push rods or anything else from the junk box. I have seen cog gears like used on the old Dutch windmills, gears sliced at an angle so the mating teeth move in and out along the axis of rotation, elliptical gears, chain and sprockets, bevel gears, worm gears and other assorted drives. On top of the platforms is a Slinky toy so it pours from one platform to the other. If y0u don't remember Slinky toys, they are the coil spring toy that would walk down stairs. Been around a long time and are still available at most any toy store for a few dollars. Use the full size Slinky and not the Slinky Junior. The platforms have to be close together. Still simpler is to make just one platform move and the other stationary, but it does not have near the appeal when operating.
The period of operation should be about 15 so operations per minute so set your reduction ratios from the engine to platform accordingly.
I have a new Slinky I picked up a couple of months ago for just this purpose.
Gail in NM,USA